Fix read_relmap_file() concurrency on Windows.

Commit d8cd0c6c95 introduced a file
rename that could fail on Windows, probably due to other backends
having an open file handle to the old file of the same name.
Re-arrange the locking slightly to prevent that, by making sure the
open() and close() run while we hold the lock.

Thomas Munro. I added an explanatory comment.

Discussion: https://postgr.es/m/CA%2BhUKGLZtCTgp4NTWV-wGbR2Nyag71%3DEfYTKjDKnk%2BfkhuFMHw%40mail.gmail.com

src/backend/utils/cache/relmapper.c

@@ -788,16 +788,6 @@ read_relmap_file(RelMapFile *map, char *dbpath, bool lock_held, int elevel)
 
 	Assert(elevel >= ERROR);
 
-	/* Open the target file. */
-	snprintf(mapfilename, sizeof(mapfilename), "%s/%s", dbpath,
-			 RELMAPPER_FILENAME);
-	fd = OpenTransientFile(mapfilename, O_RDONLY | PG_BINARY);
-	if (fd < 0)
-		ereport(elevel,
-				(errcode_for_file_access(),
-				 errmsg("could not open file \"%s\": %m",
-						mapfilename)));
-
 	/*
 	 * Grab the lock to prevent the file from being updated while we read it,
 	 * unless the caller is already holding the lock.  If the file is updated
@@ -808,6 +798,24 @@ read_relmap_file(RelMapFile *map, char *dbpath, bool lock_held, int elevel)
 	if (!lock_held)
 		LWLockAcquire(RelationMappingLock, LW_SHARED);
 
+	/*
+	 * Open the target file.
+	 *
+	 * Because Windows isn't happy about the idea of renaming over a file
+	 * that someone has open, we only open this file after acquiring the lock,
+	 * and for the same reason, we close it before releasing the lock. That
+	 * way, by the time write_relmap_file() acquires an exclusive lock, no
+	 * one else will have it open.
+	 */
+	snprintf(mapfilename, sizeof(mapfilename), "%s/%s", dbpath,
+			 RELMAPPER_FILENAME);
+	fd = OpenTransientFile(mapfilename, O_RDONLY | PG_BINARY);
+	if (fd < 0)
+		ereport(elevel,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m",
+						mapfilename)));
+
 	/* Now read the data. */
 	pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_READ);
 	r = read(fd, map, sizeof(RelMapFile));
@@ -825,15 +833,15 @@ read_relmap_file(RelMapFile *map, char *dbpath, bool lock_held, int elevel)
 	}
 	pgstat_report_wait_end();
 
-	if (!lock_held)
-		LWLockRelease(RelationMappingLock);
-
 	if (CloseTransientFile(fd) != 0)
 		ereport(elevel,
 				(errcode_for_file_access(),
 				 errmsg("could not close file \"%s\": %m",
 						mapfilename)));
 
+	if (!lock_held)
+		LWLockRelease(RelationMappingLock);
+
 	/* check for correct magic number, etc */
 	if (map->magic != RELMAPPER_FILEMAGIC ||
 		map->num_mappings < 0 ||